In a typical hard disk drive (HDD), servo sectors on a disk are used to provide position information about the location of a magnetic head over a disk surface. A common approach for writing such servo information on each disk surface on an HDD is referred to as spiral-based self-servo writing, or spiral-based SSW. According to this approach, spiral-shaped servo information (or “servo spirals”) is written on at least one disk surface prior to the SSW process.
Generally, each servo spiral is written on a disk surface to include a constant high-frequency pattern interrupted by synchronization marks that provide timing and location information to the servo system of the HDD during an SSW process. When a write head performs spiral-based SSW, the final servo information on a disk surface is written by the write head, and the position of the write head relative to the disk surface is controlled based on such servo spirals. Typically, spiral search mode is employed to demodulate the signal generated by a read head crossing the servo spirals and extract the necessary timing and location information.
In the spiral search mode, the read channel of a magnetic head is continuously seeking valid spiral crossing signals via peak detection circuitry. Thus, even when a servo spiral is offset from its expected location, the read channel can still detect the signal corresponding to the spiral crossing. However, such peak detection circuitry is not 100% accurate, because for each servo spiral, there is generally a small chance the detection circuitry will fail to detect the servo spiral or falsely determine that stray signals on the disk surface are the servo spiral. Such failures to detect and false detections typically lead to a failure of the drive, requiring the drive to be reworked or discarded.